1. Field of the Invention
The present invention relates to a sheet winding detecting device and an image forming apparatus, and more particularly to a sheet winding detecting device and an image forming apparatus that can be suitably used to detect winding of a recording sheet around a roller member such as a pressurizing roller and a fixing roller in the image forming apparatus using a thermal fixing device.
2. Description of the Related Art
There is widely used an image forming apparatus which transfers a toner image formed on a photosensitive drum onto a recording sheet, and thermally fixes the toner image on the recording sheet by a thermal fixing device having roller members such as a fixing roller and a pressurizing roller to form an image on the recording sheet. Such an image forming apparatus using a thermal fixing device separates a recording sheet bearing a fixed toner image from a roller member such as a fixing roller or a pressurizing roller by separating means such as separation pawls. If toner or stain adheres to the roller member, however, the recording sheet cannot be separated from the roller member, and this may result in winding of the recording sheet around the roller member.
Further, when an operator turns on/off a power supply of the image forming apparatus while the recording sheet is wound around the roller member, if the recording sheet wound around the roller member cannot be detected, the roller member is driven although the recording sheet is wound around the roller member. This causes the recording sheet to be further wound around the roller member.
Therefore, a sheet winding detecting device and an image forming apparatus are demanded, which reliably prevent a recording sheet from being wound around a roller member.
It is therefore an object of the present invention to provide a sheet winding detecting device and an image forming apparatus that are capable of reliably preventing a sheet wound around a conveying means that conveys the sheet from being further wound around the conveying means.
To attain the above object, in a first aspect of the present invention, there is provided a sheet winding detecting device comprising conveying means for conveying a sheet while rotating, voltage applying means for applying voltage to the conveying means, voltage detecting means for detecting voltage related to the conveying means, sheet winding detecting means for detecting whether the sheet is wound around the conveying means according to the voltage detected by the voltage detecting means, and function status detecting means for detecting whether the sheet winding detecting means is malfunctioning.
Preferably, when the function status checking means detects that the sheet winding detecting means is not malfunctioning, the voltage applying means applies voltage to the conveying means, and the sheet winding detecting means detects whether the sheet is wound around the conveying means according to the voltage applied by the voltage applying means and the detected voltage.
Preferably, the sheet winding detecting device according to the first aspect comprises control means for controlling the conveying means, and the control means controls the conveying means such that the sheet is inhibited from being conveyed when the sheet winding detecting means detects that the sheet is wound around the conveying means.
More preferably, when the function status detecting means detects that the sheet winding detecting means is malfunctioning, the voltage applying means omits to apply voltage to the conveying means.
Yet more preferably, the control means controls the conveying means such that the sheet is inhibited from being conveyed when the function status detecting means detects that the sheet winding detecting means is malfunctioning.
Alternatively, the control means controls the conveying means such that the sheet is allowed to be conveyed when the function status detecting means detects that the sheet winding detecting means is malfunctioning.
Also preferably, the sheet winding detecting device according to the first aspect comprises setting means for setting the sheet to be conveyed or to be inhibited from being conveyed when the function status detecting means detects that the sheet winding detecting means is malfunctioning, and the control means controls the conveying means according to the setting by the setting means when the function status detecting means detects that the sheet winding detecting means is malfunctioning.
In a preferred form, the conveying means has a pair of rollers that convey the sheet interposed therebetween, and the sheet winding detecting means detects whether the sheet is wound around at least one of the rollers.
Preferably, the function status detecting means is responsive to turning-on of power supply of the sheet winding detecting device, for detecting whether the sheet winding detecting means is malfunctioning.
To attain the above object, in a second aspect of the present invention, there is provided an image forming apparatus that forms an image on a recording sheet, comprising conveying means for conveying the recording sheet while rotating, the conveying means having a sheet passing area, voltage applying means for applying voltage to the conveying means, a conductive member disposed in the sheet passing area of the conveying means, voltage detecting means for detecting voltage related to the conveying means via the conductive member, holding means for holding the conductive member in contact or non-contact with the sheet passing area of the conveying means, recording sheet winding detecting means for detecting whether the recording sheet is wound around the conveying means according to the voltage applied by the voltage applying means and the voltage detected by the voltage detecting means when the conductive member is held in contact with the sheet passing area of the conveying means, and function status detecting means for detecting whether the recording sheet winding detecting means is malfunctioning.
Preferably, the function status detecting means detects whether the recording sheet winding detecting means is malfunctioning when the holding means holds the conductive member in non-contact with the sheet passing area of the conveying means.
Preferably, the image forming apparatus according to the second aspect comprises control means for controlling the conveying means, and the control means controls the conveying means such that the recording sheet is inhibited from being conveyed when the recording sheet winding detecting means detects that the recording sheet is wound around the conveying means.
More preferably, the control means controls the conveying means such that the recording sheet is inhibited from being conveyed when the function status detecting means detects that the recording sheet winding detecting means is malfunctioning.
Alternatively, the control means controls the conveying means such that the recording sheet is allowed to be conveyed when the function status detecting means detects that the recording sheet winding detecting means is malfunctioning.
Preferably, the image forming apparatus according to the second aspect comprises setting means for setting the recording sheet to be conveyed or to be inhibited from being conveyed when the function status detecting means detects that the recording sheet winding detecting means is malfunctioning, and wherein the control means controls the conveying means according to the setting by the setting means when the function status detecting means detects that the recording sheet winding detecting means is malfunctioning.
Preferably, the voltage applying means stops application of voltage when the holding means is switched between a state in which the conductive member is in contact with the sheet passing area of the conveying means and a state in which the conductive member is in non-contact with the sheet passing area of the conveying means.
Also preferably, the function status detecting means is responsive to turning-on of power supply of the image forming apparatus, for detecting whether the recording sheet winding detecting means is malfunctioning.
To attain the above object, in a third aspect of the present invention, there is provided an image forming apparatus that forms an image on a recording sheet, comprising fixing means for fixing a toner image formed on the recording sheet bearing the toner image while conveying the recording sheet, the fixing means having a rotatable member having a sheet passing area and disposed in contact with a reverse side of the recording sheet bearing the toner image, voltage applying means for applying voltage reverse in polarity to toner to the rotatable member, voltage detecting means for detecting voltage related to the rotatable member in the sheet passing area of the rotatable member, and recording sheet winding detecting means for detecting whether the recording sheet is wound around the rotatable member according to the voltage detected by the voltage detecting means.
Preferably, the image forming apparatus according to the third aspect comprises control means for controlling the fixing means, and wherein the control means controls the fixing means such that the recording sheet is inhibited from being conveyed when the recording sheet winding detecting means detects that the recording sheet is wound around the rotatable member.
Also preferably, the recording sheet winding detecting means is responsive to turning-on of power supply of the image forming apparatus, for detecting whether the recording sheet is wound around the rotatable member.
The above and other objects, features, and advantages of the invention will become more apparent from the following detailed description taken in conjunction with the following drawings.